The present invention relates to doctor blades for paper coaters, and in particular to an improved doctor blade for use with a coater that applies liquid coating material to a paper web within a pressurized application zone.
Conventional coaters of the trailing blade type include means for applying liquid coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible doctor blade, located on the trailing side of the applicator, for metering and leveling the applied coating. In general, an excess of coating material is applied onto the web, and the doctor blade meters the excess while uniformly spreading the coating onto the web surface.
It has become desirable to produce papers having a minimum amount of coating. To achieve low coat weights with conventional trailing blade equipment, it is necessary to increase the pressure of the doctor blade against the web, but that results in a high rate of wear of the blade and necessitates frequent blade replacement. High blade pressure also increases the possibility of web breaks, as well as streaking caused by foreign particles caught between the blade and web.
Conventional coaters employ a relatively long dwell or soak time, which is the time interval between initial application and doctoring of the coating. As a result, the water portion of the coating composition, as well as some of the water soluble or dispersible materials contained in the composition, migrate into the moving web at a more rapid rate than the pigment, and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive coats without intervening drying, because the successive coats tend to migrate into and contaminate the previous coat.
To overcome the disadvantages of prior applicators, and to apply lightweight coatings on paper, there has been developed a short dwell time applicator as disclosed in U.S. Pat. No. 4,250,211, issued to Damrau et al and assigned to the assignee of the present invention. In that applicator, coating material is introduced in excess into a relatively narrow application zone for being applied onto a web of paper carried through the zone. A forward wall of the applicator defines a relatively narrow gap with the web at the upstream end of the zone, and excess material in the zone overflows through the gap and forms therein a liquid seal, so that coating material in the zone and as applied onto the web is maintained under pressure. The speed of the web is adjusted for a relatively short dwell time, and a flexible doctor blade forms the downstream wall of the application zone and doctors the web at the downstream end of the zone, thereby removing excess material from and uniformly spreading the material on the web. In consequence of the short dwell time of the pressurized application of coating material onto the web, an appropriate yet lightweight amount of coating may be applied without need for high blade pressures.
A factor strongly influencing the quantity and uniformity of coating material applied onto a web, is the force and uniformity of the force with which the tip of the doctor blade is urged against the web. In short dwell time applicators, where the doctor blade forms the downstream wall of the application zone, coating liquid is against the blade and is very turbulent, especially at web travel speeds of 2500 fpm and higher. In consequence of the turbulence, there are variations in pressure of the coating liquid against the doctor blade, which cause the blade to flex and result in variations in the force with which the tip of the blade is urged against the web. Unfortunately, the variations in pressure of the coating liquid in the application zone, although small, are not controllable, and result in nonuniformities in the coating applied onto the web.